Seams are produced during garment fabrication by sewing or perhaps, in the case of thermoplastic material, welding or otherwise joining together two plies of material face side to face side along a line adjacent their edges. (It is not, of course, necessary for the face side to be distinguishable from the reverse side of either ply, although it is commonly so in textile fabrics.) The narrow strip of each ply between the sewing, welding or other joining line and the edge of the material is the "seam allowance." It is usual to press these seam allowances, as by a flat iron, in a separate pressing operation after the garment has been fabricated, the seam being opened out for this purpose so that the two joined face sides are face downwards on the ironing table, the flat iron being laid on the opened-out seam allowances to press them flat against the fabric reverse sides, so that the seam allowances do not stick up from the fabric reverse side, which would lead to bulky, unsightly seams.
Various proposals have been made to carry out this seam setting operation mechanically, continuously with the production of the seam. However, such mechanical methods have not been used to the fullest extent in garment manufacturing.